1. Technical Field
The present invention is directed toward an apparatus and method for entraining particles in a gas stream, more particularly toward a nebulizer having a nozzle generating supersonic velocities of carrier gas for entraining liquid particles in the carrier gas.
2. Background Art
A variety of compressed-gas operated nebulizers have been used for inhalation delivery of aerosols containing small particles of liquid medication to the conductive airways of the lung and the alveoli. Aerosols as used herein are relatively stable suspensions of finely divided liquid particles or solid particles in a gaseous medium, usually air or oxygen enriched air. When inhaled, aerosol particles are deposited by contact upon the various surfaces of the respiratory tract, with the goal being to deposit as great a percentage as possible of the liquid or solid particles for the desired therapeutic action or planned diagnostic activity. Inhalable aerosols are those consisting of particles smaller than 10 micrometers in equivalent diameter.
Medications deposited in the lung can readily enter the blood for action throughout the body because of the high permeability of the lung and the copious blood flow. Other medications can directly influence the efficiency of the oxygen/CO.sub.2 exchange of the lungs. Other types of aerosol particles deposited in the lung can be used as tracers of air flow or indicators of lung responses and are therefore useful as a valuable diagnostic tool.
Medical nebulizers are designed to convert water or aqueous solutions or colloidal suspensions to aerosols of fine, inhalable liquid particles that can enter the lungs of a patient during inhalation and deposit on the desired surface of the respiratory airway. One of the critical factors determining where the therapeutic aerosolized particles will be deposited within the pulmonary tree is the Mass Medium Aerosol Diameter ("MMAD"). Generally speaking, as the MMAD decreases, the site of deposition becomes more distal, up to a certain point where further decreases in the MMAD will result in particles being exhaled back out of the airway. In most medial applications, particles in the respiratory range have MMAD of 0.5-5 microns. For application of bronchodialators, MMAD ranges are preferably between 1.8-2.5 microns. For most aerosol AIDS therapies, the desired MMAD is in the range of 1 micron.
Typical pneumatic medical nebulizers feature a converging nozzle having an inlet and an outlet, with the outlet defining a primary orifice. Gas is provided to the nozzle inlet under pressure and accelerated by the converging nozzle to a subsonic, or at best, sonic velocity at the primary orifice. The high velocity of the air at the primary orifice creates a negative pressure which is used to draw liquid from a reservoir into contact with the high velocity carrier gas stream. As liquid is drawn into contact with the gas stream, it is sheared into small particles. The resulting entrained liquid/carrier gas flow is then impinged upon an impingement baffle to collect particles of greater than a select size. This type of nebulizer structure is well known in the art and representative examples include Piper, U.S. Pat. No. 5,287,847, Riggs, U.S. Pat. No. 5,355,872 and Burns, U.S. Pat. No. 3,744,722.
The nebulizers described above typically provide a fairly wide distribution of MMAD. Thus, such nebulizers do not lead to efficient delivery of aerosols of a particular MMAD. Illustrations of particle size distributions for a number of commercially available nebulizers are included in FIG. 6 of Nerbrink et al. (1994) J. Aerosol Med. 7:259. This figure illustrates relatively broad distributions, typically in ranges of 1-10 microns, with MMAD in the range of 3-9 microns. In those instances where delivery of a large percentage of particles of a select MMAD is desired, particularly a small MMAD in the range of 1 micron, such nebulizers are not well suited.
The energy of accelerated carrier gas flows in pneumatic nebulizers are typically used in two ways. First, the energy creates a low pressure zone by the increased velocity of the carrier gas which is used to draw a column of medication to be aerosolized into the flow stream. Second, once the medication has reached the flow stream, energy is expended in the shearing of the fluid into particles and carrying the suspension to the impact baffle. Higher velocity carrier gas flows, such as super sonic carrier gas flows, would make more energy available for both the shearing process as well as a greater pressure drop for drawing liquid medications into the flow stream. The increased energy for shearing should result in a larger percentage of small MMAD particles whereas the greater pressure drop will enable a higher quantity of liquid to be entrained in the carrier gas. This phenomenon is discussed in Nerbrink et al. (1994) J. Aerosol Med. 7:259.
Converging/diverging nozzles (also known as Delaval type nozzles) are known for accelerating the flow of compressed gas to a supersonic speed. For example, Fontana, U.S. Pat. No. 4,813,611, discloses a converging/diverging compressed air nozzle useful in tools for dislodging earth for excavation. Stephanoff, U.S. Pat. No. 2,297,726, teaches a converging/diverging type nozzle for accelerating a carrier gas to supersonic velocity for the purpose of drying atomized particles. Stephanoff teaches that the liquid is introduced to the throat of the converging/diverging nozzle, as illustrated in FIG. 3, or within the diverging section, as illustrated in FIG. 4. Serra Tosio, U.S. Pat. No. 5,249,740, is directed to an apparatus for humidifying instrumentation and also discloses a converging/diverging nozzle wherein liquid to be entrained is introduced into the throat of the nozzle. None of these references teach the use of a converging/diverging nozzle for creating supersonic carrier gas flows in a medical nebulizer. Moreover, in practice, the mass of the liquid introduced in the throat of the converging/diverging nozzle has been found to prevent the combination of carrier gas and entrained liquid from being accelerated to supersonic velocities. In addition, introducing liquid to the throat to the converging/diverging nozzle does not subject the liquid to the highest velocity (and therefore highest shearing energy) gas flow.
The present invention is directed to overcoming one or more of the problems discussed above.